To do this, you will need the following:
x1 decent wooden pallet with an even number of planks
A circular saw or jigsaw
A vibration sander or sandpaper and a sanding block
A screwdriver (cross or flat head depending on your choice of screws)
x4 Swivel wheels (at least 2 of these need to be braked)
x4 Metal rings on plates (optional)
You may find the crowbar useful to pry off any stubborn bits.
You can then either knock the remaining nails back out using the hammer or repeatedly bend them until they break off.
Using the saw, carefully cut through the remaining wood on top, avoiding the block of wood underneath (don't worry if you do damage the wood underneath, it won't be obvious when you have finished).
Pull out the nails using the crowbar, taking care not to pull the block off in the process.
With this done, it's time to get sanding!
This will take ages, so if you don't already have one I recommend spending money on a decent sander to speed things up.
Give it a rough going over and round off the corners, then go back over it with a finer grain to get a nice smooth finish.
If you are planning on staining and or varnishing the wood then this is the best time to do the inside.
Carefully line up the two sections and give the glue some time to dry.
Once the glue has dried, nail the sections to the blocks underneath them. You can either put the nails through the tops of the pallets or through the gaps in the wood and then tap them down with the chisel. I opted for the latter as it reduces the number of nails in the table top.
With this done, complete any varnishing or staining.